In the Battle maison (I know again) I think my game glitched.
Here's what happened:Go Link 2.0!
The opponent sent out Spiritomb! The opposing Spiritomb is exerting its pressure!
Link 2.0 used Swords Dance!
The opposing Spiritomb used Grudge!
Link 2.0 used Shadow Sneak!
The opposing Spiritomb fainted!
Ok. Here's the thing! Link 2.0 (Aegislash) only lost 1 PP a turn!
What happened?!

Edit: You may have not been paying careful attention, and misread the game, believing your Aegislash lost only 1 PP.
If you are completely sure that Aegislash only lost 1 PP, then it is most likely a glitch. It is not in the List of Glitches in X and Y, so is probably a minor one-time glitch, and is not likely to happen again.
Another possibility is a change in game mechanics, for example Ghost types are immune to Grudge, but this is extremely unlikely, as X and Y have been out for almost a year now, and would have been found, I only put this down as a possibility because there is a (very)small chance this is the case.